Vegetable oil alkyl esters intended to be used as biofuel are produced from vegetable oils obtained for example from rapeseed, sunflower, soybean or even palm. III-suited for directly feeding modern diesel engines of private cars, vegetable oils essentially consisting of triglycerides have to be converted by means of a transesterification reaction with an alcohol, methanol or ethanol for example, introduced in excess to produce vegetable oil methyl esters (VOME) and glycerin.
What is referred to as glycerol is the pure body of chemical formula C3H8O3 and glycerin or glycerin phase is understood to be a mixture predominantly containing glycerol and other impurities, such as water, methanol, mono-, di- and triglycerides for example, mono- and diglycerides being triglycerides partly converted by the transesterification reaction.
The Esterfip-H™ process developed by IFP allows to obtain a biodiesel and a glycerin of very good quality, with high yields. The flowsheet of this process consists of two fixed-bed transesterification reactors using a solid heterogeneous catalyst, operating on a continuous basis and arranged in series, which allows conversion to be maximized. The effluent from the first reactor is subjected to partial evaporation so as to remove the excess methanol introduced and thus to promote separation of the glycerin formed while favourably shifting the reaction equilibrium in order to maximize conversion in the second reactor. After the second transesterification reaction, the major part of the excess methanol is removed by evaporation (more than 99%) and recycled. The insoluble glycerin is eliminated by decantation and a final methyl ester purification stage consists in removing the soluble glycerin by passage through a column filled with a selective adsorbent. The water content of the reaction medium is controlled so as to remain below a given limit value as described in U.S. Pat. No. 6,878,837 filed by the applicant.
The current European standard EN 14,214 for biofuels sets maximum methanol, water, free glycerol, mono-, di- and triglyceride contents: 0.2% by mass for methanol, 500 mg/kg for water, 0.02% by mass free glycerol, 0.8% by mass monoglycerides, 0.2% by mass di- and triglycerides.
Free glycerol, as opposed to bonded glycerol, is defined as a glycerol molecule totally detached from any carbon chain and of formula C3H8O3.
Glycerol is referred to as bonded when the functional group of glycerol C3H8O3 is alkylated to one or more fatty acid chains giving monoglyceride, diglyceride or triglyceride molecules.
In the Esterfip-H™ process diagrammatically shown in FIG. 1 as described in the prior art, stream A at the reaction section outlet predominantly contains methyl esters, methanol, glycerol and partly converted glycerides (monoglycerides, diglycerides and triglycerides), as well as water as traces, an impurity present in the feed. The conversion reached in this reaction section (two reaction stages with an intermediate stage of separation of the glycerin coproduced) allows to obtain partial glyceride contents compatible with the European standard for biodiesel.
In the particular case of the Esterfip-H™ process, the methyl esters and the glycerol are very poorly soluble and the methanol present acts as a co-solvent. Therefore, the higher the temperature and the methanol content, the higher the glycerol content of the ester phase.
Besides, pure glycerol has a density close to 1.2 g·cm−3, whereas for the ester it is around 0.9 g·cm−3. In the presence of a small proportion of methanol, the phase predominantly containing glycerol is therefore denser than the ester phase and it thus tends to come below the latter under the effect of gravity. The ester phase thus is the supernatent phase.
Separation of the methanol from stream A coming from the reaction section (not shown in the figure) is achieved by evaporation in two stages, the second one under vacuum, in order to reach methanol and water contents allowed by the standard (zone (1) in FIG. 1), stream B corresponding to the evaporated methanol. The methanol acting as a co-solubilizing agent for the methyl esters and the glycerol, this evaporation stage makes part of the glycerol present in this stream, in a proportion ranging between 0.1 and 5% by mass, insoluble. The soluble part represents, at ambient temperature, 500 to 700 ppm mass, the allowable maximum content set by the European standard being 200 ppm mass of free glycerol. Both the insoluble glycerol and part of the soluble glycerol therefore have to be separated. This separation is then carried out in several stages.
An important problem encountered during this excess alcohol evaporation stage and consequently upon formation of a two-phase medium (ester phase with dissolved glycerol droplets and glycerin phase) is the formation of microemulsions.
A microemulsion can generally be obtained through exterior energy supply, for example by applying a very high shear to a mixture of two liquids, or by applying an ultrasonic wave to the mixture.
A microemulsion can also appear spontaneously (P. Brochette, Emulsification “Elaboration et etude des emulsions”, Techniques de l'Ingénieur, traité de Génie des procédés, J 2150), without any additional exterior energy supply, when some concentration conditions are met for each phase of the mixture. One then speaks of a spontaneous nucleation of the discontinuous phase initially dissolved in the continuous phase.
In the case of the ester phase/glycerol/alcohol ternary system obtained at the reaction section outlet of the Esterfip-H™ process, the alcohol, which is methanol, acts as a co-solubilizing agent. The progressive disappearance of the alcohol induced by a controlled evaporation leads to local oversaturation of the ester phase with glycerol. The saturation excess glycerol settles around what is referred to as nuclei consisting of surfactants: this phenomenon is called nucleation. The monoglycerides present in the effluent act as surfactants (<<Synthesis of Surfactants from Vegetable Oil Feedstocks>>, R. A. Holser, chp 10, Industrial Uses of Vegetable Oils, AOCS Press, 2005). The presence of surfactant compounds, even in very small proportions (of the order of 1 ppm), and the low interface tensions lead to the formation of very small micrometric droplets. This droplet creation occurs without requiring any other energy than the energy that tends to restore a ternary equilibrium broken as a result of the progressive disappearance, not necessarily complete, of one of the three compounds of the ternary mixture, methanol here.
The formation of microemulsions leads to the presence of a large population of glycerol microdroplets dispersed and dissolved in the ester phase.
In the Esterfip-H™ process as described in the prior art, separation of the glycerin phase occurs through gravity decantation in a decanter drum. This stage consists in sending this stream to a decanter drum 3 whose purpose is to allow the glycerin phase droplets, denser than the ester phase, to fall under the effect of gravity.
In general terms, the size of the decanter drum and the residence time of the feed in this device define the cleavage threshold of the decanter. The cleavage threshold is expressed in μm and it corresponds to the minimum drop size that can be separated by decantation in the drum. Below this threshold, the droplets do not settle rapidly enough in the decanter and they are carried along with the ester phase to the next stages of the process. Now, too long decantation times require longer effluent immobilization, thus leading to expensive overstocking and losses as regards the process profitability.
If the cleavage threshold is around 100 μm, the decantation times are fast, of the order of less than one hour (<<Extraction liquide-liquide>>, Description des appareils, J. Leybros, Techniques de l'ingénieur, Traité génie des procédés, J2764). If the cleavage threshold is below 10 μm, the decantation times become very long and the cost of the facility is significantly increased.
The decanter drum can come in form of a capacity of cylindrical shape whose axis of symmetry is horizontal. Stream D containing the ester with glycerin phase drops is injected at one end of the drum. Two outlets are arranged at the end of the drum: one is located on the upper generatrix to collect the supernatent ester phase, the other is Located at the bottom of the decanter drum to collect the glycerin phase. The ester stream containing the glycerin droplets is thus going to flow through the decanter drum horizontally from the inlet to the outlets at a velocity depending on the section and therefore on the diameter of this drum. During this horizontal flow, the glycerin drops tend to fall, under the effect of gravity, to the bottom of the decanter drum where they coalesce, i.e. they gather to form a continuous glycerin phase that can be withdrawn (stream F). The ester phase depleted in glycerin drops is withdrawn at the top of the drum (stream E).
When the microemulsion formation phenomenon that takes place during the alcohol evaporation stage is significant, separation by gravity decantation is not sufficient and the droplets of smaller size are still carried along to the next stages of the process.
In the Esterfip-H™ process, ester stream E leaving the decanter is sent to a coalescer (4). This equipment allows the glycerin droplets whose size was not large enough for decanting in the previous stage and that were consequently carried along to the decanter outlet to meet so as to form larger droplets that can then settle efficiently. Glycerin phase stream G is withdrawn at the bottom point of the coalescer. In theory, at the outlet of this equipment, ester stream H contains no more insoluble glycerin. However, too large a proportion of glycerin carried along to the decanter outlet upstream increases the coalescence difficulty and requires using a bigger equipment, which will therefore require a larger amount of steel and bigger tools. It will therefore be more expensive.
Coalescers are systems allowing the size of fine droplets to be increased by promoting the coalescence phenomenon, i.e. the formation of larger droplets (Perry's Chemical Engineers' Handbook, 7th Edition, Chp 15-17 “Liquid-liquid extraction equipment”). Once bigger, the droplets can be separated more readily by decantation for example. Coalescers are fibrous or porous solid beds whose properties are selected depending on the system to be separated. In general, cotton and glass fibers are used.
Like any industrial material, coalescers do not achieve perfect separation or they may operate under degraded working conditions (very high flow rate, aging, fouling, etc.). A proportion of fine droplets can pass through the coalescent medium. The larger the number of small-size droplets at the equipment inlet, the larger this proportion.
In order to reach the content allowed by the fuel specification, the glycerin dissolved in the ester phase still has to be separated. This stage is carried out in zone (5) by adsorption on solids, for example ion-exchange resins. These solids operate by alternating adsorption and regeneration cycles. At the end of this stage, the glycerin content of ester phase I thus meets the fuel specification (below 200 ppm).
The final ester processing chain thus comprises a decanter (3) for separating the major part of the glycerin, a coalescer (4) intended for the insoluble residual glycerin and a solid adsorption zone (5) for separating the glycerin dissolved in ester phase I. The main separation stage takes place in the decanter, whereas the stages that are conducted in the coalescer or in the solid adsorption zone are finishing stages.
In the Esterfip-H™ process as described in the prior art, the solids arranged in adsorption zone (5), ion-exchange resins for example, are in contact with part of the insoluble glycerin. Now, their use is all the more optimized as the proportion of insoluble glycerol to be separated from the ester phase is small. In the presence of too large an amount of glycerol, the solids used in the adsorption zone therefore tend to saturate more rapidly. The frequency of the adsorption/regeneration cycles increases. Regeneration is achieved using a solvent, preferably methanol. Now, repeated alternation of these cycles considerably reduces the life of these solids. For optimized operation of the solids, at the coalescer outlet, stream H should not contain more than 500 to 700 ppm mass of soluble glycerol.
The present invention thus provides a simple and improved flowsheet allowing the aforementioned drawbacks to be overcome, wherein the glycerin separation efficiency is improved while avoiding microemulsion formation during the evaporation stage. Thus, the efficiency of the decantation stage is improved. Now, this stage precisely conditions the dimensioning of the facilities required for the next stages of the process. Thus, increasing the decanter efficiency allows to reduce the size of the coalescer and to increase the efficiency thereof. The amount or the cycle time of the solids used in the adsorption zone, ion-exchange resins for example, is thus increased.